In recent years TDMA communications systems have become increasingly important, and they are expected to become even more significant in the future as they find greater use in planned satellite communications systems. In a TDMA system, multiple transmission sources at different locations communicate with a common receiver by emitting serially disposed non-overlapping information signal bursts. Time is divided into frames, and each frame typically includes a reference burst followed by the serial information bursts from the various transmitter sources. The most important application of TDMA to date is in satellite communications wherein a plurality of ground stations access a single transponder on the satellite. It has been found that TDMA possesses greater channel capacity and other advantages as compared to frequency division multiple access (FDMA).
It is recognized that an important figure of merit of any digital communications system is bit error rate, which is the ratio of the number of incorrect bits received over the link to the total number of bits received. A similar figure of merit for a continuous wave communications system is the signal to noise ratio, and signal to noise ratio, if measured in a digital system, may serve as a measure of the system bit error rate.
A known technique of measuring signal to noise ratio in a TDMA system is to transmit a known continuous wave test signal at the carrier frequency over a system which has been shut down for testing, and to separate the signal and the noise power at the receiver with appropriate filtering. A major problem with this approach is that it cannot be used for testing while normal communications are occurring, since when the continuous test signal is transmitted no traffic can be carried.